During spermatogenesis, germ cells make several important developmental decisions. One of the most important is the choice to cease mitotic proliferation and enter meiosis. Much has been learned about the switch from mitosis to meiosis in lower organisms, but the study of the molecules in the meiotic pathway and the factors which regulate them in mammalian germ cells has been more difficult. The recent development of mouse testicular cell lines, including two germ cell lines which undergo meiosis in vitro, now make molecular studies of mammalian germ cell differentiation more feasible. The experiments outlined in this proposal will test the hypothesis that Sertoli cells influence the decision of germ cells to enter meiosis. Cocultures of the somatic and germ cell lines will be used to determine the impact of Sertoli cells on the initiation of meiosis. Differential display of expressed mRNAs and recombinant antibodies will be used to identify molecules involved in the transition from mitosis to meiosis. Specific aim one will examine the effects of Sertoli cells on the initiation of meiosis by the germ cells and will identify molecules involved in the communication between the Sertoli and germ cells which potentially regulate the entry of the germ cells into meiosis. The second specific aim will identify molecules that are expressed in germ cells as they leave the mitotic cell cycle and enter the meiotic pathway. Finally, in specific aim three, recombinant antibodies will be produced to germ cell surface proteins to obtain reagents to early differentiation markers and identify additional molecules which may be involved in the regulation of germ cell differentiation. These experiments will provide insight into the molecular mechanisms responsible for mammalian germ cell differentiation.